In recent years, stroke has become a common disease being serious threat to the health of humans, especially in the elderly over the age of 50, characterized by high incidence, high morbidity, high mortality, high recurrence rate, and multiple complications, i.e. “four-highs and one-multi”. In stroke patients, intracerebral arterial stenosis, occlusion or rupture are caused by various predisposing factors, resulting in acute cerebral blood circulation disorders, clinically manifested as a transient or permanent brain dysfunction symptoms and signs.
Statistically, stroke leads to death of 40 million people over the world each year, with an annual incidence of 2 million just in China. Among the 7 million surviving patients, 4.5 million patients lose labor capability to varying degrees as well as self-care capability. The disability rate is as high as 75%. In China, 1.2 million patients die from stroke each year. Those patients who had stroke are prone to have a relapse, and the situation will become worse with each relapse. Therefore, effective means to prevent stroke recurrence are greatly needed.
Ischemic stroke accounts for about 80% of all stroke conditions, which is softening necrosis of local brain tissues due to blood circulation disorders, ischemia, and hypoxia. Its onset is mainly due to atherosclerosis and thrombosis occurring in the arteries that supply blood to the brain, causing stenosis or even occlusion, resulting in focal acute cerebral blood supply insufficiency. Also, foreign objects (solid, liquid, or gas) entering from the blood circulation into the cerebral arteries or the neck arteries that supply to the cerebral blood circulation cause blood flow obstruction or sudden decrease in blood flow volume and consequently brain tissue softening necrosis in the corresponding dominating area.
There are two major causes of ischemic brain injury: (1) due to insufficient productivity after ischemia, ATP-dependent enzyme activity and physiological activities are suppressed, chloride ions, sodium ions and water flow cause cell edema, and synaptic interstitial excitatory amino acids (mainly glutamate) accumulate, resulting in excessive activation of glutamate receptors; with increase in calcium influx mediated by NMDA and other receptors, cell depolarization due to potassium efflux, and opening of voltage-sensitive calcium channels, intracellular calcium overloads and a variety of enzymes including phospholipase and nitric oxide synthase (NOS) are excessively activated, thereby generating a series of active metabolites and free radicals and consequently causing cell damage; (2) ischemic tissues in stroke patients after being treated acquire blood perfusion or spontaneous reperfusion which inevitably lead to cerebral ischemic reperfusion injury, despite of the regaining of nutrients; in other words, although blood supply is restored at a certain time after cerebral ischemia, not only the function thereof fails to recover, but signs of more serious brain dysfunction appear.
Ischemic brain injury involves very complex pathophysiological processes, in which the interactions between the various aspects and various factors have not been fully elucidated. Nevertheless, currently the following mechanisms are considered to play an essential role in ischemic brain injury:
(1) Excitatory Amino Acid Toxicity and Ischemic Brain Damage
A large number of studies have shown that increased excitotoxicity of excitatory amino acid (EAA) during ischemia played an important role in ischemic nerve cell injury. Excitatory amino acids mainly refer to glutamate (Glu) and aspartate (Asp). The postsynaptic neurons overexcited EAA may activate intracellular signal transduction pathways, allowing some receptors to amplify the second messenger effect caused by normal physiological stimuli and triggering the expression of proinflammatory genes after ischemia. Excitatory amino acids such as Glu and Asp play a key role in ischemic nerve cell injury. The longer the ischemic duration, the higher the peak concentration of Glu and Asp in brain interstitial tissues, and the more severe the neuropathological and neurological damages, which is consistent with EAA toxicity being concentration-dependent. The toxic effects of excitatory amino acids on nerve cells are shown in various aspects: excessive EAA activates its receptors, resulting in continuous depolarization of excitatory neurons, which in turn causes intracellular Ca2+ overload and consequently lead to cell necrosis; increase in free radical (such as nitric oxide) production is promoted, and cytotoxicity is induced by the free radicals; EAA participates in a variety of metabolic processes in the brain, blocking the tricarboxylic acid cycle and decreasing ATP production, leading to increased cell toxicity by EAA.
(2) Free Radicals and Lipid Peroxidation and Ischemic Brain Damage
Ischemic brain injury is a complex pathophysiological process involving multiple factors. Generally, it is considered to be associated with tissue lipid peroxidation caused by oxygen free radicals and irreversible damage caused by intracellular calcium overload. Its detrimental effects can be summarized as: acting on polyunsaturated fatty acids, and leading to lipid peroxidation; inducing cross-linking of macromolecules such as DNA, RNA, polysaccharides, and amino acids, with the original activity or function of the cross-linked macromolecules being lost or attenuated; promoting the polymerization and degradation of polysaccharide molecules; free radicals widely attacking unsaturated fatty acid-rich nerve membranes and blood vessels, inducing a lipid peroxidation waterfall effect, resulting in protein denaturation, breaking of polynucleotide strands, and base remodification, causing damage to cell structure integrity, and seriously affecting membrane permeability, ion transportation, and membrane barrier function, thereby leading to cell death. Free radicals also evoke an increase in EAA release, leading to reperfusion injury after cerebral ischemia.
(3) Ca2+ Overload and Cerebral Ischemic Brain Injury
Ca2+ overload in ischemic brain injury is a result of the combined effects of various factors, and is a common pathway for the action of various factors in the process of cerebral ischemic injury. The impact of Ca2+ in ischemic brain injury mainly includes:
a) Mitochondrial Dysfunction: When the Intracellular and Extracellular Calcium balance is disrupted, extracellular Ca2+ flows into cells and mainly accumulate in mitochondria, and Ca2+ may inhibit ATP synthesis, impeding energy generation. Ca2+ activates phospholipases on mitochondria, causing mitochondrial membrane damage. In addition to ATP synthesis, mitochondria play an important role in cellular redox reactions and maintenance of osmotic pressure, pH value, and cytoplasmic signals, and mitochondria is the important target of cell damage.
b) Enzyme activation: Ca2+ activates Ca2+-dependent phospholipases (mainly phospholipase C and phospholipase A2) and promote membrane phospholipid degradation; the free fatty acids, prostaglandins, leukotrienes, lysophospholipids and the like that are produced in the process of membrane phospholipid degradation are toxic to cells; Ca2+ also activates calcium-dependent proteases and converts the intracellular non-toxic xanthine dehydrogenase into xanthine oxidase, with large amounts of oxygen free radicals generated; Ca2+ may activate NOS.
It has been demonstrated in experiments that the above pathophysiological changes could somehow be intervened by drugs. Compared to patients with drug withdrawal, those with long-term use of reliable drugs for prevention and treatment of ischemic stroke have their recurrence rate reduced by 80% or more and mortality reduced by 90% or more. Among patients who have taken medication for a long time over three years, 80% or more is not at risk of recurrence, and very few shows slight recurrence. This has provided a theoretical foundation for medicinally combating ischemic brain injury. Currently, commonly used drugs against cerebrovascular diseases mainly include the following categories:
NMDA receptor antagonists: antagonizing NMDA receptors, thereby inhibiting calcium influx mediated by them; a representative drug is MK801;
Calcium ion antagonists: preventing intracellular calcium overload, preventing vasospasm, and increasing blood flow; a representative drug is nimodipine;
Anti-free radicals drugs: scavenging free radicals, inhibiting lipid peroxidation, thereby inhibiting oxidative damage to brain cells, vascular endothelial cells and nerve cells; a representative drug is edaravone.
However, the specific mechanism of ischemic stroke has not been clarified and is considered to be a very complex pathophysiological process with interaction of many factors; whereas, the above drugs act by simple mechanisms, with uncertain clinical therapeutic effects or serious side effects, so that their application in the treatment of ischemic stroke is limited.
In recent years, many domestic and foreign studies have found that the anesthetic propofol may have a very positive impact on ischemic stroke. In animal and in vitro experiments, and even in some clinical studies, propofol has been proven to have significant protective and therapeutic effects on neurological impairment. It has been demonstrated in the experiments that propofol could not only block the sodium ion flow or reduce Glu release activated by potassium ions by activating the GABA receptor, but also block the inhibition of Glu transportation by glial cells after oxidative treatment, both eventually reducing extracellular Glu concentration, delaying or preventing excitatory neuron death; propofol could inhibit extracellular calcium influx through voltage-dependent calcium channels, which could increase the current inactivation rate of L-type voltage-dependent calcium channels to a certain extent, thereby reducing calcium influx; propofol could bind to GABAa receptor-specific sites, not only to increase the frequency of the opening of chloride channel by GABA, but also to enhance the binding of GABA binding sites with low affinity to GABA by positive allosteric regulation; propofol can inhibit the production of inflammatory cytokines such as TNF, IL-1 and IL-6 in the blood of patients with sepsis and had a strong inhibitory effect even at lower concentrations; propofol could inhibit the expression of the pro-apoptotic gene caspase-3 mRNA and enhance the expression of the anti-apoptotic gene Bcl-2 mRNA in brain tissues; propofol could competitively bind to membrane phospholipids, and form a stable phenoxy moiety with peroxide, which in fact forms free radicals of low activities in place of the free radicals of high activities, thereby reducing the lipid peroxidation cascade induced by the latter. The above results suggest that the mechanism by which propofol fight ischemic stroke may include anti-free radical effect, inhibition of lipid peroxidation, inhibition of intracellular calcium overload, and inhibition of cellular apoptosis. However, the clinical use of propofol in the treatment of ischemic stroke is restricted due to the general anesthetic effect thereof.